Old-fashioned speeches marks campaign of Japan's top Socialist

''Who is running Japanese politics today? Is it Mr. Nakasone?'' the speaker atop the campaign van asked. ''No,'' he replied. ''The man wielding the greatest power in Japan today is the man who was sentenced the other day to a four-year prison term.''

Thus, at the very outset of his speech, Socialist chairman Masashi Ishibashi sought to define what the opposition seeks to make the main issue of Japan's election campaign: the kingmaker role still played by a convicted criminal, former Premier Kakuei Tanaka.

The crowd in front of Kumagaya railway station seemed reasonably sympathetic. There were old and young, office workers, teachers, housewives. The Socialist Party is the underdog here in the largely rural third district of Saitama, some 50 miles northwest of Tokyo.

But the district's Socialist candidate, Tanefusa Tanami, has been a member of the local assembly for eight years. In his speeches he does not fail to remind voters how he has helped to establish new high schools or of his efforts to bring a first-class hospital to the region.

To such local rice-bowl issues, Mr. Ishibashi brings the perspectives of the Socialists' national campaign. That campaign is geared not so much to promises of what the Socialists will do as to the allegedly terrible consequences of keeping Prime Minister Yasuhiro Nakasone and his Liberal-Democratic Party in power.

The Socialists are the main opposition party, holding 107 seats in the 511 -seat House of Representatives. Their goal, Mr. Ishibashi said later, is to reach 120 seats.

Mr. Ishibashi, who entered politics 30 years ago as leader of a trade union of Japanese employees on US bases, is a more old-fashioned speaker than Mr. Nakasone. He tends to shout into the microphone rather than cultivating the conversational style of most younger politicians. He eschews the white gloves and the large white rosette the prime minister sports.

''The Socialists talk about substituting unarmed neutrality for Japan's security treaty with the United States. I just don't see how, in this world, that's an attainable policy,'' said a college junior, who will be voting for the first time.

But many feel the role played in politics by corruption-tainted men like Mr. Tanaka is a blot on Japanese democracy. They would like to register their protest short of handing over the government to the opposition.

''Let's be honest,'' a Socialist strategist says. ''The only way we will ever come to power is by persuading enough people to vote against the Liberal-Democrats rather than to vote positively for us.'' That is why Mr. Ishibashi concentrates on attacking the ruling party.

He did explain, in a meeting with foreign journalists, that the Socialists would not immediately sever Japan's US security link or abolish the Self-Defense Forces.

''Unarmed neutrality is our long-range goal,'' Mr. Ishibashi said. ''We would first have to create the international conditions that would make it feasible. What we really mean when we talk of unarmed neutrality is that we want never to have to go to war again.

''Although we want to end the military relationship with the US, we would not do so unilaterally. . . . As for political, economic, and social ties with the United States, we would want to make them even closer than before.''

Is this a realistic argument? Just asthe Socialists consider the Tanaka connection to be Nakasone's weak point, so the Liberal-Democrats gleefully fasten on the Socialists' unarmed neutrality argument to show how unrealistic and impractical, if not unpatriotic, they are.

The Tanaka factor remains the major imponderable in this election. Ishibashi says he is getting a good response to his speeches, but many voters are circumspect about expressing their feelings.

''I will listen to all the candidates,'' a young office worker said. ''I don't think I'll make up my mind until a day or so before the election.''